Composite articles incorporating honeycomb cores are commonly utilized for fabricating aerospace structures due to the advantageous combination of high strength and low weight. Honeycomb core composite articles are comprised of upper and lower fiber-reinforced, resin impregnated composite laminates that are separated and stabilized by the honeycomb core. Due to the high bending stiffness and compressive strength properties of honeycomb core composite articles, i.e., the honeycomb core functions as a shear web and separates the composite laminates from the bending neutral axis, honeycomb core composite articles have particular utility in aerospace applications such as aircraft fuselage panels and door structures. The high strength and low weight of aircraft fuselage panels and door structures fabricated as honeycomb core composite articles results in a lower overall aircraft system weight.
Honeycomb core composite articles may be fabricated utilizing black bag technology, i.e., the composite laminates and the honeycomb core are layed-up in a semi-rigid molding assembly comprised of a rigid mold member and a semi-rigid mold member, which is vacuum-bagged and co-cured under pressure and temperature in an autoclave. The lower composite laminates, the honeycomb core, and the upper composite laminates are sequentially layed-up in the semi-rigid molding assembly so that the honeycomb core is overlayed by the upper and lower composite laminates. The upper and lower composite laminates are prepregs, i.e., woven fibrous cloth, yarn, or fiber tow comprised of a matrix of orientated fibrous material such as graphite, aramids (e.g., KEVLAR.RTM., a registered trademark of E.I. du Pont de Nemours & Co., Wilmington, Del., for an aromatic polyamide fiber of extremely high tensile strength), boron, fiberglass, or the like, which is impregnated with an epoxy, phenolic, or other similar organic resinous material, and staged to form the prepreg. Film adhesive, which is applied to the honeycomb core prior to lay-up, forms the bonds between the upper and lower composite laminates and the net-shaped honeycomb core during the co-cure procedure.
Many of the aircraft fuselage panels and door structures that are fabricated as honeycomb core composite articles have configurations that include one or more ramped surfaces. Such ramped surfaces present several interrelated problems that adversely affect the fabrication and utilization of honeycomb core composite articles having ramped surfaces. First, honeycomb core material must be net shaped prior to the lay-up procedure to incorporate any such ramped surfaces, i.e., a ramped honeycomb core must be formed. Concomitantly, the honeycomb core material must be "stabilized" prior to the net shaping operation so as to preclude damage to the honeycomb core material during the net shaping operation.
Honeycomb core material is generally net shaped to incorporate ramped surfaces by machining the "stabilized" honeycomb core material to remove material, thereby forming one or more ramped surfaces. To stabilize such honeycomb core material for machining, i.e., to preclude accordioning during the machining process, film adhesive is generally applied to the upper and lower major surfaces of the honeycomb core material, and the coated honeycomb core material is cured. The cured honeycomb core material is then machined to net shape, i.e., a ramped honeycomb core.
Another layer of film adhesive is applied to the ramped honeycomb core to "prep" the ramped honeycomb core for lay-up in the semi-rigid molding assembly and subsequent co-curing. During the co-curing procedure, the second adhesive film layer forms the bonds between the upper and lower composite laminates and the ramped honeycomb core. While the foregoing fabrication techniques produce ramped honeycomb cores that are generally acceptable for black bag technology usage, such cores generally have a high total density (the phrase "total density" as used herein refers to the density of the ramped honeycomb core and the density of the film adhesive used for "stabilization" and "preparation") due to the non-optimized application of film adhesives to "stabilize" the honeycomb core material for the net shaping operation and to "prep" the ramped honeycomb core to effectuate bonding between the upper and lower composite laminates and the ramped honeycomb core during co-cure. These high total density honeycomb cores may not be acceptable for applications wherein the minimization of overall aircraft system weight is a critical design criterion.
In addition, such ramped honeycomb cores may not be sufficiently stabilized for use with present day black bag technology. Crushing, i.e., collapse, of the ramped surfaces of ramped honeycomb cores is a recurring problem during the co-curing stage of fabricating ramped honeycomb core composite articles. Crushing may occur as a result of insufficient stabilization of the ramped honeycomb core, or as a result of asymmetric pressure distributions over the ramped surfaces during the co-curing procedure, or a combination thereof. Stabilization of ramped honeycomb cores may be achieved by applying expanding adhesive foam or syntactic foam to the outer walls of the core, or utilizing expanding adhesive foam or syntactic foam to fill the core. These stabilization options, however, are not practical for applications wherein the minimization of overall aircraft system weight is a critical design criterion inasmuch as the use of expanding adhesive or syntactic foam for stabilization of ramped honeycomb cores incurs a prohibitive weight penalty. For example, one commonly utilized syntactic foam, EPOCAST.RTM., has a typical density range of about 30-50 lb/ft.sup.3.
Another option available to minimize core crushing during the co-curing procedure is to limit the co-cure pressure. As a general rule, limiting the co-cure pressure to a maximum value of about 45 psi significantly reduces the number of incidents of core crushing in fabricating ramped honeycomb core composite articles. Limiting the maximum co-cure pressure, however, is generally unacceptable for other aspects of the molding process. For example, the composite laminates of ramped honeycomb core composite articles fabricated utilizing low co-cure pressures, i.e., .ltoreq.45 psi, may embody an unacceptable level of voids, i.e., zones within the composite laminates that are insufficiently consolidated. Or, the low co-cure pressure limits may preclude complete bonding between the upper and lower composite laminates and the ramped honeycomb core. Voids and/or incomplete bonds due to low co-cure pressure limits may result in ramped honeycomb core composite articles that do not have mechanical characteristics, e.g., high strength, acceptable for use in aerospace applications.
It would be advantageous, especially for aerospace applications, to provide a co-cure molding process for fabricating honeycomb core composite articles having ramped surfaces that utilizes high co-cure pressures. To effectively utilize such high pressure co-cure molding processes, a need exists to provide ramped honeycomb cores that are optimally formed for low total density and stability in such high pressure co-cure composite molding processes. Concomitantly, a need exists to provide a composite molding apparatus for use in such high pressure co-cure molding processes utilizing low density, stabilized ramped honeycomb cores that is optimized to provide symmetric pressure distributions over the ramped surfaces of the composite laminate lay-ups and the low density, stabilized ramped honeycomb core during the co-curing stage of such molding processes.